In an image-forming device such as a color printer, when printing is conducted, color image data of RGB format which has been input is converted into image data of CMYK format which can be output by means of a lookup table for color conversion (color conversion table).
A color conversion table is formed of correspondence of an RGB value and a CMYK value, and hence can be expressed by an orthogonal grid using each correspondence as a grid point.
FIG. 28 is a schematic view of a RGB-CMYK color conversion table represented by orthogonal grids of the orthogonal coordinate system using as grid point correspondence of Rd (red), Gr (green), Bl (blue) with Cy (cyan), Mg (magenta) and Ye (yellow) as basic colors and Wh (white) and Bk (black) as achromatic color.
On each grid point of the orthogonal grids shown in FIG. 28, an RGB value and a CMY value corresponding thereto have been assigned in advance. Therefore, a CMYK value of an output device corresponding to an arbitral RBG value relating to an input device can be smoothly derived (JP-A-H09-224158).
However, it is known that the sense which can be understood by a man's brain is more sensitive to the components of three directions, i.e. the direction along the achromatic color axis which connects Wh and Bk, the radial direction centering around the achromatic color axis (chroma) and the coaxial direction (hue), rather than the axial direction of each of RGB. That is, in many cases, the feeling by which a man distinguishes the difference in color is not based only on the spatial distance, but rather is dependent on the coordinate position in a color space.
Specifically, as compared with variations in the direction along each axis of R, G and B, accuracy in the radial direction centering around the achromatic color axis or variations in distance in the coaxial direction of the achromatic color axis is a more important factor to grasp the color shade.
Therefore, in a conventional color conversion table formed of orthogonal grids in which an RGB value is simply in correspondence with a CMYK value, a human sense to colors is not reflected, and hence, the conventional color conversion table was poor in color reproducibility.
FIG. 29 is a view for explaining the problem associated with conventional color conversion tables, and is a view showing a grid point arrangement when FIG. 28 is viewed from the achromatic axis direction.
As shown in FIG. 29, in the radial direction (direction of color chroma) centering around the achromatic color axis, grid points present on the same hue are arranged at an irregular interval.
For example, between the achromatic color axis and each line of Rd, Gr, Bl, Cy, Mg and Ye, 5 grid points are arranged. However, on a line which is middle between the achromatic color axis and the Cy-Bl line, only 3 grid points are arranged.
Further, as viewed in respect of a coaxial circle, it can be understood that the interval of grid points present on the same chroma is irregular depending on the distance from the achromatic color axis (central axis). That is, it can be acknowledged that the variation in a reproducible hue varies depending on the difference in chroma.
Therefore, according to such conventional color conversion table, it was difficult to conduct color adjustment such as optimization of chroma according to hue or optimization of hue according to chroma.
Under such circumstances, in order to solve the problems associated with conventional color conversion table, a radial grid color conversion table has been proposed, in which an appropriate color conversion process is enabled by adding necessary grid points to a conventional color conversion table (JP-A-2009-17097 and JP-A-2009-17098).
FIG. 30 is a grid point arrangement view when a radial color conversion table disclosed in JP-A-2009-17097 or JP-A-2009-17098 is viewed from the achromatic color axis direction.
As shown in FIG. 30, according to this radial color conversion table, grid points are arranged at an equal interval according to a certain hue width, and at the same time, grid points are arranged at an equal interval according to a certain chroma width. Accordingly, an adequate color adjustment which conforms to shade or tone sensed by a human being is possible.